Dr. Rick Sacra, 51, infected with Ebola on mission in Liberia

This undated photo from the family shows Dr. Richard Sacra in Liberia. On Wednesday, Sept. 2, 2014, the president of the SIM missionary group said Dr. Sacra was the third U.S. aid worker sickened with Ebola; he decided to return to Liberia after the two others fell ill with the deadly virus. (AP Photo/Doug Sacra)

The third American sickened with Ebola is a Boston-area doctor who decided to return to Liberia after the two others fell ill with the deadly virus, the president of his missionary group said Wednesday.

Dr. Rick Sacra went back to Liberia about a month ago and was not caring for Ebola patients, instead delivering babies at the missionary group's hospital in Liberia, SIM President Bruce Johnson said at a news conference.

When a fellow SIM worker and another missionary doctor became sick, Sacra called and said, "I'm ready to go," Johnson said.

SIM needed doctors to take care of patients not infected with Ebola, said Will Elthick, director of SIM's operations in Liberia. Sacra followed all protocols to protect himself, Elthick said, and he's in good spirits and able to email.

SIM does not know whether Sacra, who is in isolation, will return to the U.S. for treatment, as the other two Americans did, Johnson said.

Dr. Bruce Ribner, who oversaw the first two missionaries' treatment at Emory University Hospital in Atlanta, told NBC's "Today" show that he also does not know whether the third patient will come there.

"I know there have been discussions that this person will be coming back to the United States," Ribner, head of the hospital's infectious disease unit, said. "I don't believe the actual site where they're coming back has been decided yet."

Dr. Kent Brantly, the first Ebola patient to arrive at Emory, said he knew the latest American to fall ill quite well and has prayed for him and his family, whom he said were "holding up pretty well." Brantly worked for Samaritan's Purse, a missionary group that partners with SIM in Liberia. Both are based in North Carolina.

Nancy Writebol, the second American sickened, said she believes an experimental drug, her medical care and her faith helped save her.

"Those were some very, very dark days," Writebol, 59, said of her illness.

She said didn't think she had Ebola but she was tested for it anyway. She said she didn't know whether she would live or die: "I had no clue what was going to happen."

The Writebols left their home last year for their missionary work. At the clinic in Liberia, Writebol's duties included disinfecting staff entering or leaving the Ebola treatment area.

On July 22, she said, she was feeling tired. She had had malaria before and tested positive for it again. After finishing her medicine, she said, she still had symptoms.

She didn't think she had Ebola but she was tested for it anyway. Her husband, David, broke the news to her that she tested positive, she said.

He took a step toward her to hug her, but Writebol said she stopped him in his tracks, not wanting to infect him. "I said, `no, it's going to be OK," she said.

Brantly was first flown to Atlanta in a specially equipped jet, and Writebol followed.

They were both released last month. Writebol has been spending time with her husband at an undisclosed location. Her husband was quarantined for a week at the SIM campus before being released.

The virus that has killed more than 1,500 people in West Africa is spread by direct contact with blood or bodily fluids, not through casual contact.